Within low bit rate audio coding often the available bit rate is too low to model an entire spectrum of an audio signal with a deterministic type of encoder, such as a sinusoidal or a waveform encoder. Two approaches have been used to overcome this problem.
According to one approach bandwidth of the signal to be modeled is limited such that the available bit rate is sufficient to model the limited bandwidth with the deterministic encoder. A disadvantage of this approach is that the necessary bandwidth limitation is effectively a reduction in audio quality.
According to a second approach the entire bandwidth is modeled. Part of the signal is modeled with the deterministic encoder using a large portion of the available bit rate and the remaining parts of the audio signal are modeled with noise. This often leads to reasonable results because the perceived bandwidth and timbre of the original audio signal is nearly maintained. However, regarding the second mentioned approach a problem is to determine how the noise signal should be generated.
When a sinusoidal encoder is used as a deterministic encoder, often a residual signal, i.e. a signal that is left after subtracting the sinusoidal components in each audio segment, is used as a basis for estimating noise parameters. Many advanced encoders prepare the residual signal before noise parameter estimation to overcome some artefacts such as an overly noisy sound quality of the decoded signal or low frequency artefacts due to poor spectral resolution of the noise encoder. An example on such approach is seen in WO 2004049311.
When a waveform encoder is used, e.g. a transform encoder, the encoder decides which audio bands should not or can not be modeled by the transform encoder. Information about these omitted bands is then transmitted so as to allow the decoder to generate noise accordingly.
The above described methods suffer from the disadvantage that already at the encoder side final decisions have to be made about the noise signal that is going to be generated at the decoder side. As a consequence, it is not permitted that parameters or data for the deterministic part of the decoder are changed once the signal has been encoded. This may happen for example during transmission of the encoded signal or during fast rescaling of a compressed audio file where certain layers of information are dropped. If this is done, the consequence will be that, at the decoder side, the generated noise signal will not match the resulting signal from the deterministic decoder part and considerable audible artefacts can be the result. In other words, noise coding according to the described principles is not scalable because it does not allow modifications to the deterministic signal after noise parameters have been estimated.